Phillips submitted his resignation as senior vice president, chief financial officer and treasurer "to pursue an expanded opportunity in the industry," the company said. Phillips will step down Nov. 10 and provide financial, accounting and transition consulting services to Novavax (NASDAQ: NVAX) through the end of the year.

Novavax CEO Stan Erck will act as interim CFO during the search for Phillips' replacement. “Buck has been instrumental in a number of significant Novavax accomplishments," Erck said in a statement about his planned departure. The company declined to comment further.

Phillips has been a visible player in Novavax's executive leadership, handling communications functions and, despite his financial background, demonstrating an intricate understanding of the company's science in interviews and on company financial calls. Novavax share prices dipped slightly Friday to $1.15 by 1 p.m., down 3 percent from a previous close of $1.18.

After the failed trial, Novavax returned that RSV vaccine to a phase 2 clinical trial and changed its dosing approach. In August, company leaders said that change showed a benefit in patients, but said they planned to embark on yet another phase 2 trial in 2018 — news that sent the company's stock price spiraling again. Analysts worried about the delays from another trial, but Phillips has said in the past Novavax continues to believe it will be first to market with an RSV vaccine for the elderly.

In the meantime, Novavax has also refocused on its flu vaccine candidate, announcing plans to enter into clinical trials that pit its product against pharmaceutical giant Sanofi's Fluzone High-Dose. Novavax hopes to see that data by the end of this year.

Buck joined Novavax in June 2013. Before that, he was CFO of Rockville bioscience company Micromet Inc., which was working on an experimental leukemia drug when it was acquired by Amgen in a $1.6 billion deal in 2012. He was also formerly a managing director of Vector Fund Management and a biotech analyst and director of venture investments at Invesco Funds Group Inc.